1. Field of the Invention
The present invention relates to a display device, in particular, an organic electroluminescence (EL) display device including an organic EL element.
2. Description of the Related Art
In recent years, an organic EL display device including an organic EL element as a self-luminous device has been attracting attention as a flat-panel display. Here, the organic EL element is an electronic element having a pair of electrodes and an organic compound layer placed between the pair of electrodes, and the organic EL display device is a display device using the organic EL element as a display element.
In a top-emission type display device, a lower electrode to be provided closer to a substrate side with respect to an organic compound layer is required to have a high reflectance for improving the light extraction efficiency of the device. A metal material having a high reflectance such as aluminum or silver is used as a material for the lower electrode, and the material is formed into a layer having a thickness of 30 nm or more in order that the reflectance intrinsic to the material may be obtained. When an organic EL element is formed by laminating the organic compound layer and an upper electrode on the lower electrode having a thickness of 30 nm or more, the organic compound layer becomes thin at end portions of the lower electrode (referring to an upper end portion, a lower end portion, and a side wall portion). The application of a voltage between the lower electrode and upper electrode of such organic EL element causes a phenomenon in which a current concentrates on an end portion of the lower electrode. As a result, the following problem arises. The emission intensity of the organic compound layer at the end portion becomes larger than its emission intensity at a flat portion except the end portion, and hence local brightness unevenness occurs in the same light-emitting pixel.
Several methods have heretofore been proposed as methods for solving the problem. Japanese Patent Application Laid-Open No. 2003-332082 discloses a display device in which all upper end portions of lower electrodes are roundly chamfered so that an electric field may not concentrate in the upper end portions. In addition, Japanese Patent Application Laid-Open No. 2005-327694 discloses a display device in which a side surface of a lower electrode is tilted by processing an upper end portion of the lower electrode into a taper shape.
However, even in the case where an upper end portion of a lower electrode is processed like Japanese Patent Application Laid-Open No. 2003-332082 and Japanese Patent Application Laid-Open No. 2005-327694, as in the case where the upper end portion is not processed, the thickness of an organic compound layer to be formed on the lower electrode tends to be smaller at the upper end portion than at a flat portion as the upper surface of the lower electrode. Accordingly, such a tendency that a current is more likely to concentrate in the upper end portion of the lower electrode than in the flat portion of the lower electrode does not change. In addition, the concentration of the current in the upper end portion of the lower electrode makes the emission intensity of the layer at the upper end portion larger than its emission intensity at the flat portion of the lower electrode. Accordingly, the problem of the local brightness unevenness in the same light-emitting pixel still remains.
In addition, the current efficiency and chromaticity of an organic EL element largely change depending on the thickness of an organic compound layer constituting the element. Accordingly, when the thickness of the organic compound layer to be formed on a lower electrode is not uniform owing to the state of the lower electrode, actually causing a current to flow through the organic compound layer may cause a reduction in luminous efficiency of the element or the deterioration of its color purity.